The present invention relates generally to manufacture of integrated circuits, and more particularly to minimization and stabilization of the temperature coefficient of nichrome resistors formed on integrated circuits.
It is well known that the resistances of nichrome thin film resistors formed on integrated circuit wafers undergo substantial shifts. The TCR (temperature coefficient of resistance) of the nichrome material is one of the ways in which this shift can be quantified. The nichrome material is a nickel chromium oxide that is deposited as a thin film on the semiconductor wafer during the wafer fabrication process and then baked in a nitrogen-oxygen environment to cause the sheet resistance of the film to attain a certain value. After wafer fabrication is complete, the wafer is transferred to an electrical testing facility. After electrical testing, the wafers are transferred to a packaging facility wherein the individual integrated circuit chips (die) are separated and packaged before final testing. Many integrated circuits are designed with a requirement that the TCR of nichrome resistors thereon be less than a specified maximum value.
The largest shift in the TCR of nichrome resistors usually occurs during the packaging of individual integrated circuit chips after the integrated circuit wafer fabrication and laser trimming of the nichrome resistors is complete. The TCR of a particular wafer fabrication process used by the present assignee is usually roughly 30 ppm (parts per million)/xc2x0 C. The above described TCR shifts in the nichrome resistors usually are positive shifts. The amount of the shift due to the packaging environment typically is highly variable from day to day (and even from hour to hour), for reasons that are neither well understood nor presently controllable, apparently because the xe2x80x9cinteractionsxe2x80x9d of the nichrome material with various aspects of the wafer fabrication processes and the subsequent packaging processes are highly variable.
If the TCR shifts could be reliably minimized or made slightly negative, this would make it possible to greatly reduce the number of wafers rejected at the end of the wafer fabrication process, and therefore would make it possible to greatly reduce the economic loss associated with the rejection of such wafers.
The above described TCR shift may be very problematic because wafer fabrication facilities usually establish a certain specification for the maximum acceptable TCR of the nichrome resistors of a particular integrated circuit product.
In the known wafer fabrication process, which is subsequently described in more detail with reference to the process flowchart of FIG. 4, the TCR of the nichrome resistors of each wafer is measured before the wafers are transferred to the electrical testing facility. If the measured TCR of the nichrome on a semiconductor wafer, which typically includes hundreds of integrated circuit chips, exceeds a maximum specified TCR (referred to herein as TCRMAX), the entire wafer must be scrapped consequently, the very large economic cost that has been incurred in fabricating the wafer up to that point is lost.
The TCR measurements are made after the wafers are baked or stabilized prior to the passivation process. The values of sheet resistance of the nichrome material on the wafers are probed and measured at 45 degrees Centigrade. Then the wafers are heated and the sheet resistance values are measured again at 145 degrees Centigrade. The value of the TCR then is computed from the two sheet resistance measurements. If the computed value of the TCR of the wafer is not within the specified limits established for the integrated circuits under consideration, the wafer will be rejected.
RF plasma sputter etching techniques are commonly utilized in manufacture of integrated circuit wafers to pre-clean wafers prior to metal deposition on integrated circuit wafers being manufactured. See xe2x80x9cSilicon Processing for VLSI Eraxe2x80x9d by S. Wolf, Process Integration, 2000, Page 108, Lettuce Press. RF plasma sputter etching is a process wherein an inductively coupled power supply provides energy to a coil wound around a reactor chamber to produce a medium frequency plasma of argon in a reaction chamber. RF power is applied to a chuck which supports a wafer and produces an RF bias that causes the Ar+ ions to impinge on the surfaces of the wafer and remove contaminant material therefrom.
Thus, there is an unmet need for a procedure which lowers the TCR of nichrome resistors and which can be easily incorporated in standard or conventional integrated circuit wafer manufacturing processes.
There also is an unmet need for a procedure which lowers and stabilizes the TCR of nichrome resistors and which can be easily incorporated in standard integrated circuit wafer manufacturing processes.
It is an object of the present invention to provide a method for lowering the TCR of nichrome resistors.
It is another object of the present invention to provide a method for lowering the TCR of nichrome resistors, which method can be easily incorporated in a standard integrated circuit wafer manufacturing process.
It is another object of the present invention to provide a method for lowering and/stabilizing the TCR of nichrome resistors, which method can be easily incorporated in a standard integrated circuit wafer manufacturing process.
It is another object of the present invention to provide a method for lowering/stabilizing the TCR of nichrome resistors by a fixed amount.
It is another object of the present invention to provide a method for lowering/stabilizing the TCR of resistors other than nichrome resistors.
Briefly described, and in accordance with one embodiment, the present invention provides a method for processing a partially fabricated semiconductor wafer having a layer of resistor material patterned to form a plurality of resistors on a surface of the wafer, the method including performing a wet pre-metallization cleaning step on the surface of the wafer, performing an RF plasma sputter etching process on the surface of the wafer in the first reactor, advancing the wafer from the first reactor into a second reactor while maintaining unbroken vacuum conditions in the first and second reactors, and depositing a layer of metal on the surface of the wafer in the second reactor. The metal then is patterned to form a predetermined metal interconnection pattern. A stabilization bake cycle then is performed on the wafer. The TCR of the resistor material is measured, and the wafer is rejected if the measured TCR is greater than a predetermined value, but otherwise fabrication of the wafer is completed. In the described method, the resistor material is composed of nichrome. Argon gas is passed into the first reactor with the wafer therein, and an inductively coupled power supply produces argon plasma in the first reactor adjacent to the surface of the wafer. An RF bias signal is applied to the wafer to cause argon ions to impinge on the surface of the wafer and remove contaminant material therefrom. The RF plasma etching is performed for approximately 15-30 seconds. with the wafer at a temperature of approximately 400 degrees Centigrade. The RF signal has a voltage of approximately 100 volts and a frequency of approximately 13.5 MHz and causes the wafer to attract the argon ions. The argon plasma is generated by means of an inductive coil wound around a reaction chamber of the first reactor by applying a medium frequency power supply signal having a frequency of approximately 100 kHz across the inductive coil.